A thin film superconducting wire including a laminated structure containing a superconducting layer is conventionally known. The thin film superconducting wire is wound longitudinally in a spiral manner around the surface of a cylindrical core material called a former, for example, to thereby implement a superconducting cable conductor. To achieve cost reduction, it is requested to reduce the outer diameter of a cross section that longitudinally intersects the superconducting cable conductor, to reduce the outer diameter of a cross section that longitudinally intersects the former, and to wind the thin film superconducting wire around the surface of the former. Accordingly, it is requested to wind the thin film superconducting wire while bending it with a large curvature relative to its longitudinal direction. Thus, during the winding, a larger bending stress is applied to the radially outer side (toward the outer peripheral side) of the former in the thin film superconducting wire.
For example, a plurality of cable cores constituting a superconducting cable disclosed in Japanese Patent Laying-Open No. 2006-331893 (Patent Document 1) indicated below has the following structure. Specifically, each cable core is configured to include a former positioned at the center of cable core, a first superconducting layer made of a superconducting wire wound around the outer periphery of the former (a superconducting wire with a superconducting filament covered by a stabilizing metal such as silver), an insulation layer located outside the first superconducting layer, and a second superconducting layer located outside the insulation layer. At a cross section that longitudinally intersects the cable core, the second superconducting layer located at the outer side has a diameter larger than that of the first superconducting layer located at the inner side. Thus, a bending stress applied to the first superconducting layer while winding the cable core around the surface of the former or insulation layer, for example, has a larger value than that applied to the second superconducting layer. Therefore, executed in Patent Document 1 is a process of making the tensile strength of the second superconducting layer to which a large bending stress is applied greater than that applied to the first superconducting layer by making the tensile strength of a material constituting the second superconducting layer greater than that of a material constituting the first superconducting layer (more specifically, by a method such as producing a covering layer such as a copper plating layer as a reinforcing member that covers the surface of the second superconducting layer, or bonding a tape layer as a reinforcing member to the second superconducting layer). The cable core and the superconducting cable are thereby improved in bending properties.